Platform noise mitigation in OFDM receivers

ABSTRACT

Techniques and structures are presented for use in mitigating the effects of platform noise within a wireless device.

TECHNICAL FIELD

The invention relates generally to wireless communication and, moreparticularly, to wireless networking.

BACKGROUND OF THE INVENTION

Wireless networking transceivers have traditionally been concerned morewith interference from external sources than with internally derivedinterference. However, studies have shown that a significant amount ofinterference experienced in wireless networking receivers is generatedwithin the wireless platform itself. In a notebook computer, forexample, various clocks and other components were found to generatesignals that can cause significant interference within a wirelessnetworking receiver within the system. This platform noise is typicallyfrequency dependent and can degrade communication performance in theassociated wireless network. Techniques and structures for mitigatingplatform noise in wireless receivers are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example wireless networkarrangement that may incorporate features of the present invention;

FIG. 2 is a block diagram illustrating a portion of an example receiverin accordance with an embodiment of the present invention;

FIG. 3 is a block diagram illustrating a portion of an example receiverin accordance with another embodiment of the present invention;

FIG. 4 is a block diagram illustrating a portion of an example receiverin accordance with still another embodiment of the present invention;

FIG. 5 is a flowchart illustrating an example method for use inmitigating platform noise within a wireless device in accordance with anembodiment of the present invention;

FIG. 6 is a block diagram illustrating a portion of an example receiverthat may be used in a MIMO based system in accordance with an embodimentof the present invention; and

FIG. 7 is a block diagram illustrating a portion of an example receiverthat may be used in a MIMO based system in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

FIG. 1 is a block diagram illustrating an example wireless networkarrangement 10 that may incorporate features of the present invention.As illustrated, a first wireless device 12 is communicating with asecond wireless device 14 through a wireless communication link. Thefirst and second wireless devices are each capable of communicatingusing orthogonal frequency division multiplexing (OFDM) techniques. Thefirst and second wireless devices 12, 14 may include, for example, (a) awireless access point (AP) and a wireless client device acting in aninfrastructure mode of operation, (b) two wireless client devices actingin an ad hoc mode of operation, (c) two wireless APs communicating withone another, or (d) some other network arrangement. As illustrated, thefirst wireless device 12 has multiple antennas 16 for use intransmitting signals into and/or receiving signals from the wirelesschannel. Likewise, the second wireless device 14 includes multipleantennas 18 for use in transmitting signals into and/or receivingsignals from the wireless channel. Any type of antennas may be usedincluding, for example, one or more dipole antennas, one or more patchantennas, one or more helical antennas, and/or others.

As shown in FIG. 1, the second wireless device 14 may include, amongother things, a receiver 20 coupled to the multiple antennas 18 for usein processing signals received by the multiple antennas 18. Although notshown, the receiver 20 may include a radio frequency (RF) portion toprocess the RF receive signals and convert them to a basebandrepresentation and a baseband processing portion to further process thebaseband information. The output of the receiver 20 may be delivered to,for example, a medium access control (MAC) processing subsystem 22 forfurther processing. Transmitter functionality may also be providedwithin the second wireless device 14. If provided, the transmitterfunctionality may use the same antennas 18 as the receiver 20 (with, forexample, a duplexer or other structure to permit antenna sharing) ordifferent “transmit” antennas may be provided. In the illustratedembodiment, the receiver 20 and a lower portion of the MAC processingsubsystem 22 (the lower MAC) are located within a wireless networkinterface card (NIC) and an upper portion of the MAC processingsubsystem 22 is located within a host device that is carrying thewireless NIC. The wireless NIC may be removably inserted into a port(e.g., a peripheral component interconnect (PCI) slot, a PC card slot,etc.) of the host device or it may be an integral part of the hostdevice. The host device may include, for example, a computer unit (e.g.,a desktop, laptop, palmtop, tablet, etc. computer), a personal digitalassistant (PDA), a cellular telephone, or some other device. Otherarrangements within the second wireless device 14 may alternatively beused.

In addition to the upper portion of the MAC subsystem 22, the hostdevice may include other components and subsystems for storing,manipulating, and/or displaying data. Some of these structures may besources of spurious signal energy that can appear within the operationalbandwidth of the receiver 20. This spurious energy can operate asinterference within the receiver 20 that can degrade the overallcommunication performance of the device and will be referred to hereinas platform noise. Some sources of platform noise within a host devicemay include, for example, display clocks driving LCD displays, CK410clocks, PCI clocks, PCI Express clocks, USB clocks, Azalea codec clocks,system management clocks, and/or others. The present invention relatesto methods and structures that may be used to mitigate the effects ofplatform noise (and other non-white noise sources) within a wirelessnetwork device.

FIG. 2 is a block diagram illustrating a portion of an example receiver30 in accordance with an embodiment of the present invention. Thereceiver 30 may be used, for example, within the second wireless device14 of FIG. 1 or within other wireless devices and systems. Asillustrated, the receiver 30 may include one or more of: a noise powerper tone estimator 32, an antenna selector 34, a serial to parallelconverter 36, a fast Fourier transform (FFT) 38, a phase correction,demodulation, and soft bit calculation unit 42, a channel estimator 44,a pilot tracking unit 46, a Viterbi decoder 48, a frequency domainpacket detector 50, and a memory 52. The receiver 30 is coupled to atleast two antennas. In the discussion that follows, the receiver 30 willbe discussed in the context of a dual antenna system, although it shouldbe appreciated that additional antennas may also be used. The noisepower per tone estimator 32 estimates the platform noise power per OFDMtone (e.g., σ²/tone) for each of the two antennas. The platform noiseper OFDM tone may be estimated during, for example, a protocol quietperiod when no on-air transaction is taking place. The antenna selector34 selects one of the available antennas for use during subsequentcommunication activities based on, in at least one embodiment, a signalto interference plus noise ratio (SINR) associated with the twoantennas, using the platform noise per tone information. A receivesignal from the selected antenna is subsequently directed to the inputof the serial-to-parallel converter 36 which coverts the serial streamto a group of parallel samples for delivery to the FFT 38. Samples thatare associated with a cyclic prefix of a received OFDM symbol may bediscarded. In another approach, a separate cyclic prefix removal unitmay be provided before the serial to parallel converter 36 to remove thecyclic prefix before conversion to parallel.

The FFT 38 converts the time based samples output by the serial toparallel converter 36 to a frequency domain representation. Althoughillustrated as an FFT, it should be appreciated that any discreteFourier transform functionality may be used. The complex output samplesof the FFT 38 thus represent the received signal 40 of the receiver 30(i.e., y=hx+n for each tone). The frequency domain packet detector 50monitors the output of the FFT 38 to determine when a packet has beenreceived by the receiver 30. As will be described in greater detail, thefrequency domain packet detector 50 may be used to reduce or eliminatethe occurrence of packet detections that are “false alarms” in thereceiver 30. The channel estimator 44 receives the output samples fromthe FFT 38 and uses the samples to generate a channel estimate h for thewireless channel. The channel estimate information may then be deliveredto the phase correction, demodulation, and soft bit calculation unit 42for use in calculating the soft bits for the Viterbi decoder 48. Thepilot tracking unit 46 receives the symbols associated with the pilottones of the received signal y for use in pilot tracking. The pilottracking unit 46 then causes the phase correction, demodulation, andsoft bit calculation unit 42 to perform phase corrections based on thepilot tracking results. As shown, the phase correction, demodulation,and soft bit calculation unit 42 also receives the platform noise pertone information from the noise power per tone estimator 32. The phasecorrection, demodulation, and soft bit calculation unit 42 uses theplatform noise per tone information to weight the various tones based onthe corresponding noise power during generation of the soft bits to bedelivered to the Viterbi decoder 48. The Viterbi decoder subsequentlyuses the soft bits to determine the data stream that the received signaly most likely represents.

As discussed previously, the noise power per tone estimator 32 estimatesthe platform noise per OFDM tone (e.g., σ²/tone) for each of the twoantennas. This information may then be directed to the antenna selector34 for use in selecting an antenna for subsequent communicationactivity. As will be described in greater detail below, the platformnoise per tone information for the selected antenna may also bedistributed to the phase correction, demodulation, and soft bitcalculation unit 42, the pilot tracking unit 46, and the frequencydomain packet detector 50 for use in performing their respectivefunctions. It has been found that, in many cases, the platform noisespectrum is relatively static over time. Thus, in at least oneembodiment of the present invention, the platform noise per toneinformation is only occasionally or periodically updated. A memory 52may be included within the receiver 30 to store the platform noise pertone information for use by the various elements of the receiver 30 inthe interim period between estimates. This may reduce processing latencyby removing the need to estimate the noise power spectrum for eachindividual receive operation. In other embodiments, no such memory isprovided.

As described above, the antenna selector 34 selects one of two (or more)available antennas for use during subsequent communication activities,based on SINR. To determine the SINR values associated with the twoantennas, the antenna selector uses the platform noise per toneinformation generated by the noise power per tone estimator 32. In atleast one embodiment of the present invention, the following metric isused for antenna selection:

$\begin{matrix}{M_{k} = {\sum\limits_{i = 1}^{52}{{\log_{2}\left( {1 + \frac{S_{i}^{2}}{\sigma_{i}^{2}}} \right)}\mspace{14mu} k\;{th}\mspace{14mu}{antenna}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where S_(i) is the signal strength associated with the ith tone andσ_(i) ² is the noise variance associated with the ith tone. The metricM_(k) is for use in an OFDM system that includes 52 tones. The abovemetric is calculated for each available antenna and the antenna havingthe highest metric value is selected. Other SINR-based metrics mayalternatively be used.

As described above, the frequency domain packet detector 50 monitors theoutput of the FFT 38 to determine when a packet has been received by thereceiver 30. After detecting a packet, the frequency domain packetdetector 50 may cause the remainder of the receiver 30 to be madeavailable to process the packet. Wireless packets transmitted within anIEEE 802.11 based system will typically have a number of short preamblesat a beginning of the packet that each include a known data pattern of apredetermined duration (e.g., 8 microseconds). In past receivers, packetdetection was performed in the time domain using a sliding windowcorrelation approach to detect the short preambles. This technique maybe represented as follows:

${m(t)} = {\sum\limits_{l = 0}^{N - 1}{{s\left( {t + l} \right)}{s^{*}\left( {t + l - N} \right)}}}$where m(t) is the correlation metric, s(t) is the time sample, N is theperiodicity of the preamble, and s*(t) denotes the complex conjugate ofs(t). The metric m(t) would then be compared to a threshold value todetermine whether a packet had arrived (e.g., m(t)>m₀ means a packet wasreceived). The above-described technique, however, can result in thegeneration of “false alarms” if the noise within the received energy hasthe same or a similar periodicity to the short preamble (e.g., aperiodicity of around 8 microseconds). The frequency domain packetdetector 50 also uses a sliding window correlation approach, but in thefrequency domain so that tones having a higher level of platform noisemay be discounted during the packet detection process. The frequencydomain version of the correlation metric above may be expressed as:

${m(t)} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{{\overset{\sim}{S}\left( {t,k} \right)}{{\overset{\sim}{S}}^{*}\left( {{t + T},k} \right)}}}}$where

${\overset{\sim}{S}\left( {t,k} \right)} = {\sum\limits_{l = 0}^{N - 1}{{s\left( {t + l} \right)}{{\mathbb{e}}^{j\; k\; 2\pi\;{l/N}}.}}}$The frequency domain packet detector 50 uses a modified version of thisexpression to take the platform noise into account and thus reduce falsealarms. The modified version may be expressed as follows:

$\begin{matrix}{{m(t)} = {\frac{1}{N}n^{2}{\sum\limits_{k = 0}^{N - 1}{{\overset{\sim}{S}\left( {t,k} \right)}{{{\overset{\sim}{S}}^{*}\left( {{t + T},k} \right)}/{\sigma^{2}(k)}}}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$where

$\frac{1}{n^{2}} = {\sum\limits_{k = 0}^{N - 1}\frac{1}{\sigma^{2}(k)}}$is the normalization. Other frequency domain packet detection metricsthat take the platform noise per tone information into account to weightthe tones may alternatively be used.

A subset of the tones within an OFDM symbol are typically pilot tones.Pilot tones may be used by a receiver of an OFDM symbol to perform phasecorrections so that a receive clock associated with the receiver moreclosely tracks a transmit clock within the transmitter of the OFDMsymbol. Each pilot tone will typically include a unique symbol thatallows the pilot tone to be recognized by the receiver. Interference onthe pilot tones can be very problematic for the receiver. That is,interference on one or more pilot tones may adversely affect the phasecorrection of the receiver to such an extent that the entire OFDM symbolmay be corrupted. Interference on a data tone, on the other hand, willtypically only affect the reception of the information associated withthat tone. Thus, in at least one embodiment of the present invention,the pilot tones of a received OFDM symbol are weighted using thecorresponding noise information to discount the noisier tones from thepilot tracking calculation. In an IEEE 802.11 based system, for example,an OFDM symbol may have 52 tones from a frequency of −26 to −1 and +1 to+26 with pilot tones at frequencies −14, −7, +7, and +14. To perform thepilot tracking, these tones may be weighted using the platform noise pertone information generated by the noise power per tone estimator 32(which, in at least one embodiment, is stored in memory 52). Forexample, the pilot tones may be weighted as follows:

$\left. y_{({{- 14},{- 7},7,14})}\Rightarrow\frac{y_{({{- 14},{- 7},7,14})}}{\sigma} \right.$where σ is the standard deviation of the per tone platform noise. Otherweighting techniques may alternatively be used.

The soft bits calculated by the phase correction, demodulation, and softbit calculation unit 42 represent the inputs that are delivered to theViterbi decoder 48. In at least one embodiment of the present invention,the soft bits are weighted by the inverse of the noise variance pertone. This may be expressed as follows for each individual tone:

(b_s) = sym 2bit(y, h, σ²)${\hat{x} = {\underset{x \in {QAMconstellation}}{\arg\mspace{11mu}\min}{{y - {hx}}}^{2}}};$b_h_(i) = QAM 2bit(x̂), i = 1, …  , N;${{b\_ s}_{i} = {{\log\left( \frac{P\left( {{b\_ h}_{i} = \left. 1 \middle| y \right.} \right)}{P\left( {{b\_ h}_{i} = \left. 0 \middle| y \right.} \right)} \right)}\left( \sigma^{2} \right)}},{i = 1},\ldots\mspace{11mu},{N;}$where b_s represents a soft bit, b_h represents a hard bit, sym2bit is asymbol to bit operator, h is the channel matrix for the tone,P(b_h_(i)=1|y) is the probability that the ith hard bit is 1 given y,P(b_h_(i)=0|y) is the probability that the ith hard bit is 0 given y,and σ² is the noise variance for the tone. For quadrature phase shiftkeying (QPSK), the last expression above reduces to:

${b\_ s}_{1} = {\frac{1}{2\sigma^{2}}{{Re}\left( {yh}^{*} \right)}}$${b\_ s}_{2} = {\frac{1}{2\sigma^{2}}{{Im}\left( {yh}^{*} \right)}}$Similar expressions apply for other modulation coding schemes.

In the embodiment described above, per tone noise weighting for softbits is performed by dividing by the noise variance for each tone todiscount the noisy tones. Other weighting methods may alternatively beused. In one possible alternative, a binary weighting technique is used.That is, if the inverse of the noise variance for a particular tone isgreater than a threshold value, then a weight of 1 is used for the tone;otherwise, a weight of 0 is used. This technique ignores tones that areat or above a given noise level and only considers tones that are belowthe given noise level. In another possible approach, a binary shift formof weighting may be used to discount noisy tones, where the weight for aparticular tone is rounded off to the nearest 2^(n), n being an integer.This may be expressed as follows:

$w = 2^{{nint}{\lbrack{\log_{2}{(\frac{1}{\sigma^{2}})}}\rbrack}}$where

$\frac{1}{\sigma^{2}}$is the noise variance for the tone and nint[x] is the nearest integeroperator. This approach has been found to be comparable in performanceto division by the noise variance, but at a significantly lowercomplexity. In addition, because the platform noise spectrum has beenfound in many cases to be relatively static, the weighting calculationsmay be performed once (or periodically) and stored to reduce overallcomplexity. Similar weighting techniques may be used for antennaselection, packet detection, and pilot tracking.

FIG. 3 is a block diagram illustrating a portion of an example receiver60 in accordance with an embodiment of the present invention. Thereceiver 60 is similar to the receiver 30 of FIG. 2, except for themanner in which the per tone noise weighting is performed. That is, inthe receiver 30 of FIG. 2, the per tone noise weighting was performed aspart of the soft bit calculation. In the receiver 60 of FIG. 3, on theother hand, the per tone noise weighting is performed as part of thechannel estimation. As illustrated, the receiver 60 may include one ormore of: a noise power per tone estimator 32, an antenna selector 34, aserial to parallel converter 36, a fast Fourier transform (FFT) 38, aphase correction, demodulation, and soft bit calculation unit 62, achannel estimator 64, a pilot tracking unit 46, a Viterbi decoder 48, afrequency domain packet detector 50, and a memory 52. The noise powerper tone estimator 32, the antenna selector 34, the serial to parallelconverter 36, the fast Fourier transform (FFT) 38, the pilot trackingunit 46, the Viterbi decoder 48, the frequency domain packet detector50, and the memory 52 each operate in substantially the same mannerdescribed above in connection with FIG. 2. The channel estimator 64receives the output samples from the FFT 38, as before, and uses thesamples to generate a channel estimate for the wireless channel.However, the channel estimator 64 also receives the noise power per toneinformation from the noise power per tone estimator 32 (or memory 52)and uses the information to calculate

$h_{1} = \frac{h}{\sigma^{2}}$for each of the tones. The channel estimator 64 may then transfer the hand h₁ matrices for each tone to the phase correction, demodulation, andsoft bit calculation unit 62, which uses the matrices to generate thesoft bits.

By performing the noise weighting on the estimated channels, instead ofduring the soft bit calculation, a significant reduction incomputational complexity may be achieved. For example, thousands ofadditional multiplications will typically need to be performed toimplement per tone noise weighting during soft bit calculation, whilenoise weighting on the estimated channels may only need the same numberof multiplications as the number of tones being used (e.g., 52 in oneembodiment). Table 1 below sets out the soft bit calculation expressionsand corresponding computational complexity for the 16QAM coding schemeusing per tone noise weighting in the softbit calculation(sym2bit(y,h,σ²)), per tone noise weighting of the channel estimates(sym2bit(y,h,h₁,1)), and no per tone noise weighting where the noise isconsidered constant across the tones (sym2bit(y,h,1)). As shown, thecomputational complexity of the soft bit calculation is the same for theper tone noise weighting of the channel estimates as it is for thesituation where no per tone noise weighting is performed. Thecalculation of |H|² and h*h₁ were not counted in the complexitydetermination since they remain the same for all soft bits over one toneand can be pre-calculated.

TABLE 1 sym2bit(y, h, σ²) Complexity sym2bit(y, h, 1) Complexitysym2bit(y, h, h₁1) Complexity b_s_(0,n) 1/σ² Re(h * y_(n)) 2 multsRe(h * y_(n)) 1 mult Re(h₁ * y_(n)) 1 mult b_s_(1,n) 2|h|²/σ² −|b_s_(0,n)| 1 mult + 2|h|² − |b_s_(0,n)| 1 adder 2 h * h₁ − |b_s_(0,n)|1 adder 1 adder b_s_(2,n) 1/σ² Im(h * y_(n)) 2 mult Im(h * y_(n)) 1 multIm(h₁ * y_(n)) 1 mult b_s_(3,n) 2|h|²/(σ² − |b_s_(2,n)|) 1 mult + 2|h|²− |b_s_(2,n)| 1 adder 2 h * h₁ − |b_s_(2,n)| 1 adder 1 adder

FIG. 4 is a block diagram illustrating a portion of an example receiver70 in accordance with an embodiment of the present invention. Thereceiver 70 is similar to the receiver 30 of FIG. 2, except for themanner in which the per tone noise weighting is performed. That is, inthe receiver 70 of FIG. 4, the per tone noise weighting is performedjust after the FFT. As illustrated, the receiver 70 may include one ormore of: a noise power per tone estimator 32, an antenna selector 34, aserial to parallel converter 36, a fast Fourier transform (FFT) 38, anoise power per tone weighting unit 72, a phase correction,demodulation, and soft bit calculation unit 76, a channel estimator 78,a pilot tracking unit 80, a Viterbi decoder 48, a frequency domainpacket detector 82, and a memory 52. The noise power per tone estimator32, the antenna selector 34, the serial to parallel converter 36, thefast Fourier transform (FFT) 38, the Viterbi decoder 48, and the memory52 each operate in substantially the same manner described above inconnection with FIG. 2.

As described above, the complex output samples of the FFT 38 representthe received signal of the receiver (i.e., y=hx+n for each tone). Thenoise power per tone weighting unit 72 applies a noise weighting factorto the received symbol y on each tone using the noise power per toneinformation received from the noise power per tone estimator 32 (ormemory 52). This makes it appear that the modified received signal 74(i.e., y′=h′x+n′ for each tone) has a relatively flat noise spectrumacross the tones. The remainder of the processing within the receiver 70may therefore be the same as, or similar to, the receive processingwithin a receiver that assumes a flat noise spectrum. For example, thefrequency domain packet detector 82 may perform frequency domain packetdetection using a sliding window correlation approach without performingindividual noise weighting on the tones and will still be able to reduceor eliminate the occurrence of false alarms. Similarly, the pilottracking unit 80 will not have to individually weight the pilot tonesfor use in phase correction activities as the weighting has already beenperformed. The channel estimator 78 will output the modified channelmatrix h′=h/σ, instead of the actual matrix h. The phase correction,demodulation, and soft bit calculation unit 76 may utilize the same softbit calculation expressions that would be used for the situation whereno per tone noise weighting is performed (e.g., see Table 1 for theexpressions used for 16QAM). However, the modified channel matrix h′ isused in the expression, rather than the actual matrix h.

In the embodiments described above, the noise per tone informationgenerated within the noise power per tone estimator 32 was used toperform the antenna selection, packet detection, pilot tracking, andsoft bit calculation functions. It should be appreciated, however, thatone or more of these functions may be performed in a conventionalfashion (i.e., not using platform noise per tone information) in variousalternative embodiments of the invention. For example, in somealternative embodiments, the antenna selector 34 of FIGS. 2, 3, and 4 isreplaced with an antenna selector that selects an antenna based onreceived signal power. Similarly, in other alternative embodiments, thefrequency domain packet detector 50, 82 of FIGS. 2, 3, and 4 is replacedwith a time domain packet detector that does not take noise power pertone into account. Other modifications and variations are also withinthe scope of the present invention.

FIG. 5 is a flowchart illustrating an example method 90 for use inmitigating the effects of platform noise within a wireless device inaccordance with an embodiment of the present invention. First, platformnoise power per tone is estimated during a protocol quiet period (block92). This platform noise power per tone information may be estimatedwithin, for example, an on-board platform noise spectrum analyzer. Theplatform noise per tone information is then stored within a memory(block 94). In at least one embodiment, the memory is periodicallyupdated by the platform noise analyzer. The platform noise per toneinformation is eventually used to select one of a plurality of antennasfor use by the wireless device (block 96). In at least one embodiment, ametric is calculated (e.g., see Equation 1) for each of the availableantennas and an antenna having the best (e.g., highest) metric value isselected. The platform noise per tone information is then used toperform frequency domain packet detection in a manner that can reducethe occurrence of packet detection false alarms in the wireless device(block 98). This frequency domain packet detection may utilize a slidingwindow correlation approach in the frequency domain to generate a metric(e.g., see Equation 2). A packet may be considered to be detected whenthe metric value exceeds a threshold level. The platform noise per toneinformation may also be used to perform pilot tracking in the wirelessdevice to reduce timing errors caused by platform noise. This may beaccomplished by weighting the pilot tones using the noise per toneinformation during pilot tracking. The platform noise power per toneinformation may also be used in generating soft bits for delivery to aViterbi decoder within the wireless device (block 102). As describedpreviously, the soft bits may be calculated in a number of differentways using the platform noise per tone information (see FIGS. 2, 3, and4).

FIG. 6 is a block diagram illustrating a portion of an example receiver110 that may be used in a MIMO based system in accordance with anembodiment of the present invention. The receiver 110 may be used, forexample, within the second wireless device 14 of FIG. 1 or within otherwireless devices and systems. In the previously described embodiments,one of a number of different antennas was selected to be used duringcommunication activity. In the receiver 110 of FIG. 6, the signal andnoise components associated with multiple receive antennas are combinedin a manner that reduces or eliminates the noise using MMSE techniques.As before, in the discussion that follows, a dual antenna arrangement isassumed. It should be appreciated that systems having more than twoantennas may also be implemented.

As shown in FIG. 6, the receiver 110 may include one or more of: a noisepower per tone estimator, 112, a correlation matrix calculator 114, apacket detector 116, an FFT 118, a minimum mean square error (MMSE)channel equalizer 122, a soft bit calculator 124, and a Viterbi decoder126. The noise power per tone estimator 122 is coupled to each of theavailable receive antennas and estimates platform noise power per tonefor each antenna. These estimates may be performed during, for example,a protocol quiet period. The correlation matrix calculator 114 thencalculates a correlation matrix R for the two antennas (for each tone).This may also be performed during the protocol quiet period. In at leastone embodiment, the correlation matrix R may be calculated as follows:

$R = \begin{bmatrix}\sigma_{1}^{2} & {{\rho\sigma}_{1}\sigma_{2}} \\\left( {{\rho\sigma}_{1}\sigma_{2}} \right)^{\dagger} & \sigma_{2}^{2}\end{bmatrix}$where a σ₁ ² and σ₂ ² are the average noise powers for one tone at themain and auxiliary antennas, respectively, ρ is the correspondingcorrelation coefficient, and (ρσ₁σ₂)^(†) is the conjugate transpose of(ρσ₁σ₂). The correlation coefficient ρ may be calculated as follows:

$\rho = \frac{{< n_{1}},{n_{2} >}}{\sqrt{{< n_{1}},{n_{1} > < n_{2}},{n_{2} >}}}$where n₁ and n₂ are noise sequences from the first and second antenna,and σ₁ ²=<n₁,n₁>. After the correlation matrix R has been determined, itmay be used to calculate the MMSE equalization response to be used bythe MMSE channel equalizer 122 as follows:G=H ⁵⁵⁴(HH ^(†) +R/σ _(x) ²)⁻¹where G is the MMSE equalization response, H is the channel matrix, H⁵⁵⁴is the conjugate transpose of the channel matrix, R is the correlationmatrix, and σ_(x) ² is the transmit power. The channel H may beestimated (for each tone) using the MMSE algorithm as follows:Ĥ=X ^(†)(XX ⁵⁵⁴ +R)⁻¹ Ywhere Y is the received signal (Y=Hx+N) and X=diag(x,x).

The packet detector 116 detects when a packet has been received by thereceiver 110. After a packet has been detected, the packet detector 116may open up the remainder of the receiver 110 to process the receivedpacket. In the illustrated embodiment, packet detection is performed inthe time domain using a sliding window correlation approach. However, inother embodiments, a frequency domain packet detection scheme may beemployed, as discussed previously. The FFT 118 converts a received OFDMsymbol from a time domain representation to a frequency domainrepresentation. Although not shown, cyclic prefix removal will typicallybe employed. The frequency domain output of the FFT 118 isrepresentative of the received signal 120 (i.e., Y=Hx+N for each tone).The MMSE channel equalizer 122 operates on the received signal Y, usingthe equalization response discussed above, to generate an estimate ofthe transmitted data x for the tone. This may be performed for each toneof the OFDM symbol.

The equalization performed within the MMSE channel equalizer 122 willoften leave some residual noise within the resulting signal x. Thisnoise may be further reduced by calculating soft bits and then inputtingthe soft bits to a Viterbi decoder. The soft bit calculator 124generates soft bits for each tone based on the estimated data x. As inprevious embodiments, the soft bit calculator 124 may utilize platformnoise variance per tone information in the soft bit calculation. BecauseMMSE equalization has been performed, the soft bit calculator 124utilizes the “residual” platform noise per tone (after MMSEequalization) in the calculation. In at least one embodiment, theresidual noise variance per tone is calculated as follows:Σ² =GRG ^(†)where G is the MMSE equalization response and R is the correlationmatrix. The Viterbi decoder 126 receives the soft bits calculated by thesoft bit calculator 124 and uses then to determine the data most likelytransmitted to the receiver 110.

Maximum Ratio Combining (MRC) is a technique that involves weighing thereceived signals at multiple receive antennas by their respectivechannel responses and then combining the signals. When the noise powerat two receive antennas is equal and uncorrelated, MRC is optimal formaximizing output signal to noise ratio (SNR). Conversely, when thenoise is not equal and is correlated, MRC is sub-optimal. In at leastone embodiment of the present invention, a decorrelator is first used ina receive chain to decorrelate the platform noise received at tworeceive antennas. This essentially whitens the platform noise in amanner that removes the correlated portion of the noise and allows MRCto be implemented in an optimal or near optimal manner. FIG. 7 is ablock diagram illustrating a portion of an example receiver 130 inaccordance with an embodiment of the present invention. As illustrated,the receiver 130 may include one or more of: a noise power per toneestimator 112, a correlation matrix calculator 114, a packet detector116, an FFT 118, a decorrelator 132, a maximum ratio combining (MRC)unit 134, and a Viterbi decoder 126. The noise power per tone estimator112, the correlation matrix calculator 114, the packet detector 116, theFFT 118, and the Viterbi decoder 126 are substantially the same as thosediscussed in connection with the receiver 110 of FIG. 6.

The decorrelator 132 uses the correlation matrix R calculated by thecorrelation matrix calculator 114 to decorrelate the received signal Y.In at least one embodiment, this may be carried out using the followingequation:Y′=R ^(−1/2) Ywhere Y′ is the decorrelated receive signal and R is the correlationmatrix. After the decorrelator 132, the effective noise N′ (i.e.,N′=R^(−1/2)N) becomes decorrelated with equal power. The equivalentchannel becomes H′=R^(−1/2)H. The received signals may now be input tothe MRC 134 with the optimal performance conditions satisfied. The MRC134 send the combined information to the Viterbi decoder 126, which usesthe information to identify the data that was most likely transmitted tothe receiver 130.

The techniques and structures of the present invention may beimplemented in any of a variety of different forms. For example,features of the invention may be embodied within laptop, palmtop,desktop, and tablet computers having wireless capability; personaldigital assistants (PDAs) having wireless capability; cellulartelephones and other handheld wireless communicators; pagers; satellitecommunicators; cameras having wireless capability; audio, video, andmultimedia devices having wireless capability; network interface cards(NICs) and other network interface structures; integrated circuits; asinstructions and/or data structures stored on machine readable media;and/or in other formats. Examples of different types of machine readablemedia that may be used include floppy diskettes, hard disks, opticaldisks, compact disc read only memories (CD-ROMs), magneto-optical disks,read only memories (ROMs), random access memories (RAMs), erasableprogrammable ROMs (EPROMs), electrically erasable programmable ROMs(EEPROMs), magnetic or optical cards, flash memory, and/or other typesof media suitable for storing electronic instructions or data. In atleast one form, the invention is embodied as a set of instructions thatare modulated onto a carrier wave for transmission over a transmissionmedium.

It should be appreciated that the individual blocks illustrated in theblock diagrams herein may be functional in nature and do not necessarilycorrespond to discrete hardware elements. For example, in at least oneembodiment, two or more of the blocks are implemented in software withina single (or multiple) digital processing device(s). The digitalprocessing device(s) may include, for example, a general purposemicroprocessor, a digital signal processor (DSP), a reduced instructionset computer (RISC), a complex instruction set computer (CISC), a fieldprogrammable gate array (FPGA), an application specific integratedcircuit (ASIC), and/or others, including combinations of the above.Hardware, software, firmware, and hybrid implementations may be used.

In the foregoing detailed description, various features of the inventionare grouped together in one or more individual embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects may lie in less thanall features of each disclosed embodiment.

Although the present invention has been described in conjunction withcertain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art readily understand.Such modifications and variations are considered to be within thepurview and scope of the invention and the appended claims.

1. A method comprising: estimating platform noise power per toneinformation during a quiet period in a wireless device; receiving anorthogonal frequency division multiplexing (OFDM) symbol from a wirelesschannel; and using said platform noise power per tone information togenerate soft bits for said OFDM symbol for delivery to a Viterbidecoder within said wireless device, wherein using said platform noisepower per tone information to generate soft bits includes: convertingsaid OFDM symbol from a time domain representation to a frequency domainrepresentation that includes a number of frequency domain data samples,each of said frequency domain data samples representing a received datasymbol for a corresponding tone of said OFDM symbol; weighting saidfrequency domain data samples using said platform noise power per toneinformation; and delivering said weighted samples to a soft bitcalculation unit for use in calculating soft bits, wherein said soft bitcalculation unit assumes flat noise across the OFDM tones.
 2. The methodof claim 1, further comprising: storing said platform noise power pertone information in a memory after estimating, wherein using saidplatform noise power per tone information includes retrieving saidplatform noise power per tone information from said memory.
 3. Themethod of claim 2, further comprising: repeating estimating and storingsaid platform noise power per tone information at predefined intervals.4. The method of claim 1, further comprising: using said platform noisepower per tone information to select one of a plurality of antennas foruse in subsequent communication based on signal to interference plusnoise ratio (SINR).
 5. The method of claim 1, further comprising: usingsaid platform noise power per tone information to perform frequencydomain packet detection using a sliding window correlation in thefrequency domain to detect a packet when received by said wirelessdevice, wherein using said platform noise power per tone information toperform frequency domain packet detection includes discounting tonesbased on noise power level.
 6. The method of claim 1, furthercomprising: using said platform noise power per tone information toperform pilot tracking with pilot tones within said OFDM symbol, whereinusing said platform noise power per tone information to perform pilottracking includes discounting pilot tones based on noise power level. 7.The method of claim 1, wherein: using said platform noise power per toneinformation to generate soft bits includes: generating channel estimatesfor said wireless channel using said frequency domain samples; weightingsaid channel estimates using said platform noise power per toneinformation to generate weighted channel estimates; and delivering saidweighted channel estimates to a soft bit calculation unit for use incalculating soft bits.
 8. An apparatus comprising: a noise power pertone estimator to estimate platform noise power per tone during a quietperiod when no communication activity is occurring; a discrete Fouriertransform (DFT) unit to convert a received orthogonal frequency divisionmultiplexing (OFDM) symbol from a time domain representation to afrequency domain representation, said frequency domain representationincluding a number of frequency domain samples, each of said frequencydomain samples representing a received data symbol for a correspondingtone of said OFDM symbol; a soft bit generation subsystem to generatesoft bits for said received OFDM symbol using said platform noise powerper tone information generated by said noise power per tone estimator; aViterbi decoder to process said soft bits; and a frequency domain packetdetector, coupled to receive an output of said discrete Fouriertransform unit, to detect the reception of a packet by said apparatususing a sliding window correlation in the frequency domain, saidfrequency domain packet detector to utilize said platform noise powerper tone information estimated by said noise power per tone estimator inperforming the sliding window correlation to reduce the occurrence ofpacket detect false alarms within said apparatus.
 9. The apparatus ofclaim 8, further comprising: a memory to store said platform noise powerper tone information estimated by said noise power per tone estimator,wherein said soft bit generation subsystem retrieves said platform noisepower per tone information from said memory when needed to generate softbits.
 10. The apparatus of claim 8, wherein: said soft bit generationsubsystem includes: a noise power per tone weighting unit to weight saidfrequency domain samples using said platform noise power per toneinformation to generate weighted samples; and a soft bit calculationunit to calculate said soft bits using said weighted samples, whereinsaid soft bit calculation unit assumes flat noise across the OFDM tones.11. The apparatus of claim 8, wherein: said soft bit generationsubsystem includes a soft bit calculation unit to receive said platformnoise power per tone information at an input thereof and to calculatesaid soft bits using said platform noise power per tone information. 12.The apparatus of claim 8, wherein: said soft bit generation subsystemincludes: a channel estimator to generate channel estimates for saidwireless channel using said frequency domain samples and to weight saidchannel estimates using said platform noise power per tone informationto generate weighted channel estimates; and a soft bit calculation unitto receive said frequency domain samples, said channel estimates, andsaid weighted channel estimates for use in generating said soft bits.13. The apparatus of claim 8, further comprising: an antenna selector toselect one of a plurality of antennas to use during subsequentcommunication activity, said antenna selector to use platform noisepower per tone information estimated by said noise power per toneestimator to select said antenna based on signal to interference plusnoise ratio (SINR).
 14. The apparatus of claim 8, further comprising: apilot tracking unit, coupled to receive outputs of said discrete Fouriertransform unit that are associated with pilot tones of an OFDM receivesymbol, to perform pilot tracking for said apparatus, said pilottracking unit to utilize said platform noise power per tone informationestimated by said noise power per tone estimator to discount pilot tonesthat have a high level of platform noise.
 15. A system comprising: atleast two dipole antennas; a noise power per tone estimator to estimateplatform noise power per tone during a quiet period when nocommunication activity is occurring; a discrete Fourier transform (DFT)unit to convert a received orthogonal frequency division multiplexing(OFDM) symbol from a time domain representation to a frequency domainrepresentation, said frequency domain representation including a numberof frequency domain samples, each of said frequency domain samplesrepresenting a received data symbol for a corresponding tone of saidOFDM symbol; a soft bit generation subsystem to generate soft bits forsaid received OFDM symbol using said platform noise power per toneinformation generated by said noise power per tone estimator; a Viterbidecoder to process said soft bits; and a frequency domain packetdetector, coupled to receive an output of said discrete Fouriertransform unit, to detect the reception of a packet by said apparatususing a sliding window correlation in the frequency domain, saidfrequency domain packet detector to utilize said platform noise powerper tone information estimated by said noise power per tone estimator inperforming the sliding window correlation to reduce the occurrence ofpacket detect false alarms within said apparatus.
 16. The system ofclaim 15, further comprising: a memory to store said platform noisepower per tone information estimated by said noise power per toneestimator, wherein said soft bit generation subsystem retrieves saidplatform noise power per tone information from said memory when neededto generate soft bits.
 17. The system of claim 15, wherein: said softbit generation subsystem includes: a noise power per tone weighting unitto weight said frequency domain samples using said platform noise powerper tone information to generate weighted samples; and a soft bitcalculation unit to calculate said soft bits using said weightedsamples, wherein said soft bit calculation unit assumes flat noiseacross the OFDM tones.
 18. The system of claim 15, wherein: said softbit generation subsystem includes a soft bit calculation unit to receivesaid platform noise power per tone information at an input thereof andto calculate said soft bits using said platform noise power per toneinformation.
 19. The system of claim 15, wherein: said soft bitgeneration subsystem includes: a channel estimator to generate channelestimates for said wireless channel using said frequency domain samplesand to weight said channel estimates using said platform noise power pertone information to generate weighted channel estimates; and a soft bitcalculation unit to receive said frequency domain samples, said channelestimates, and said weighted channel estimates for use in generatingsaid soft bits.
 20. An article comprising a storage medium havinginstructions stored thereon that, when executed by a computing platform,operate to: acquire platform noise per tone information for a wirelessdevice; receive an orthogonal frequency division multiplexing (OFDM)symbol from a wireless channel; and use said platform noise power pertone information to generate soft bits for said OFDM symbol for deliveryto a Viterbi decoder within said wireless device, wherein operation touse said platform noise power per tone information to generate soft bitsincludes operation to: convert said OFDM symbol from a time domainrepresentation to a frequency domain representation that includes anumber of frequency domain data samples, each of said frequency domaindata samples representing a received data symbol for a correspondingtone of said OFDM symbol; weight said frequency domain data samplesusing said platform noise power per tone information; and deliver saidweighted samples to a soft bit calculation unit for use in calculatingsoft bits, wherein said soft bit calculation unit assumes flat noiseacross the OFDM tones.
 21. The article of claim 20, wherein: operationto acquire platform noise per tone information for a wireless deviceincludes operation to retrieve said platform noise power per toneinformation from a memory within said wireless device.
 22. The articleof claim 20, wherein said instructions further operate to: select one ofa plurality of antennas associated with said wireless device for use insubsequent communication based on signal to interference plus noiseratio (SINR) using said platform noise power per tone information. 23.The article of claim 20, wherein said instructions further operate to:perform frequency domain packet detection to detect an incoming packetusing a sliding window correlation in the frequency domain that usessaid platform noise power per tone information to discount tones basedon noise power level.
 24. The article of claim 20, wherein saidinstructions further operate to: perform pilot tracking with pilot toneswithin a received OFDM symbol using said platform noise power per toneinformation to discount pilot tones based on noise power level.
 25. Thearticle of claim 20, wherein: operation to use said platform noise powerper tone information to generate soft bits includes operation to:generate channel estimates for said wireless channel using saidfrequency domain samples; weight said channel estimates using saidplatform noise power per tone information; and process said frequencydomain samples, said channel estimates, and said weighted channelestimates to generate said soft bits.